The ideal Schottky diode in an integrated circuit would be one with low forward drop, low leakage, low capacitance and high breakdown. In general, this dictates a large area device to obtain the low forward drop by lowering the resistance due to spreading resistance effects, contact resistance and some depletion edge resistance. However, the increase in size normally results in greater leakage, higher capacitance, and lower breakdown.
Leakage is the result of: 1) the large area required to reduce the resistance, 2) the resultant increase in edges, and depletion region of the larger junction, 3) surface effects caused by contamination that will normally increase due to the increase in area, 4) the sharp junction curvature to the surface that gives high reverse leakage at elevated voltages, 5) lower breakdown voltage, and 6) junction spreading as the voltage is increased, resulting in increased depletion region and its resultant increased leakage.
To offset these issues, it is common practice to invoke the following:
1) MOS-type practices must be used in the fabrication of the device in order to guarantee a good clean, non contaminated surface.
2) The metal on the oxide is overlapped over the metallic junction to provide a more ideal surface termination of the junction. (This is applicable at low operating voltages, but at higher voltages the sharp electrode edge will result in high leakage, as well as lowering the breakdown voltage. FIG. 1A illustrates a conventional Schottky diode 10 with a metal overlap 12. This metal overlap 12 improves the characteristics when relatively high reverse breakdown voltages are encountered. This is shown in FIG. 1B.)
3) The sharp electrode edge problem is reduced by having a guard ring 82 around the periphery of the metallic junction. FIG. 1B shows the guarding structure 82. This guard ring also prevents the spread of the depletion region and therefore the added depletion volume.
Another approach that reduces the junction curvature and therefore helps to reduce the leakage is using an oxide isolated approach using an isoplanar type oxide. FIG. 2 illustrates an isoplanar type Schottky diode 20. Note that the oxide curvature shown at 22 helps to soften the junction curvature and termination. This approach has some issues with leakage due to the junction tending to move out and therefore create a larger depletion region which increases the “capture” leakage. This approach therefore usually ends up with a guard ring around it to both soften the junction curvature plus prevent the spread of the depletion region.
Accordingly, what is needed is a system and method for providing an ideal Schottky diode that has a lower resistance, controlled capacitance and reduced leakage. The present invention addresses such a need.